The present invention relates to light emitting diodes. As well understood by those of ordinary skill in this art, in its most basic form a light emitting diode is formed of one or more semiconductor materials that includes at least one p-n junction (the diode) and which emits light (photons) of a particular color when current passes (is injected) through the device.
Because light emitting diodes are formed from semiconductor materials, they represent one group of “solid state” devices; i.e., those electrical or electronic devices formed in solid compositions, and that operate without the use of flow of electrons through a gas or a vacuum that characterized a much earlier generation of electronic equipment such as vacuum tubes. In an increasingly large number of electronic applications, solid state devices are overwhelmingly preferred because of their relatively low cost, high reliability, small size, light weight, and the derivative advantages that these provide.
In particular, light emitting diodes have become almost ubiquitous in their appearance in devices of all types. In recent years, the availability of light emitting diodes that will emit in the blue portion of the visible spectrum has expanded yet again the available applications for light emitting diodes. In addition to providing blue light per se, blue LEDs with the appropriate wavelengths (about 455-492 nanometers), can be incorporated with LEDs of the other primary colors (red and green, both of which have generally been more widely available than blue) to form multiple combinations of visible colors for many purposes. Indeed, the availability of all three primary colors in light emitting diodes has opened the possibility for solid state production of white light (i.e., the combination of all the primary colors), and such devices are increasingly available in the consumer marketplace as well as other areas of commerce.
As is further understood by those of skill in this art, the color that an LED produces is based on a number of factors, but primarily depends upon the bandgap of the semiconductor material being used, often combined with various doping schemes, including compensated doping schemes. The material being used is the fundamental factor, however, because the material's full bandgap represents the limiting factor in the energy transitions that can produce a photon. Thus, materials with smaller bandgaps cannot produce photons having sufficient energy (and corresponding wavelength and frequency) to fall into the higher energy (blue and violet) portion of the visible spectrum. In particular, in order to produce a blue photon, a material must have a band gap of at least 2.5 eV (e.g. for a 492 nm photon), and only a relatively few semiconductor materials meet this criteria. Among these are the Group III nitrides, silicon carbide (SiC), and diamond.
Although much interest and success in blue LEDs has focused upon silicon carbide based devices, Group III nitrides have raised more recent and greater interest because of their characteristics as direct rather than indirect emitters. In somewhat simplistic terms, a direct emitter produces a photon that incorporates all of the energy of the bandgap transition, while an indirect emitter emits some of the transition energy as a photon and some as vibrational energy (a phonon). Thus, a direct transition is more efficient than an indirect one in an LED. Additionally, the bandgap of Group III nitride materials can be tailored somewhat by the atomic composition of the nitride. Thus, blue light emitting diodes are generally formed in combinations of gallium nitride, aluminum nitride, indium nitride, and various ternary and tertiary versions of these materials. In particular, indium gallium nitride is an attractive candidate because its bandgap can be tuned by adjusting the amount of indium present.
Although the blue LED has expanded the universe of LED applications, its use can be to some extent limited in producing white light for other, more mundane reasons. For example, in order to produce white light from the red-green-blue combination, a lamp or pixel must incorporate a red LED, a blue LED and a green LED. Additionally, producing the necessary circuitry and physical arrangements to house and operate three LEDs is more complex than for single-color LEDs when they are incorporated into devices.
Accordingly, recent interest has focused upon the use of single color LEDs in combination with fluorescent and phosphorescent materials to produce desired colors from single LEDs. Although many materials respond in fluorescent or phosphorescent fashion to light in the visible spectrum, and thus will respond to visible LEDs, more tend to respond to the higher-energy photons in the ultraviolet portion of the spectrum. Furthermore, certain visible LED-phosphor combinations raise particular disadvantages. For example, a relatively high energy photon from a blue LED will produce phosphorescence in a number of materials, including phosphorescence of white light. Because the blue LED is stimulating the phosphorescence, however, the light always tends to have a blue component in it that may be undesired in a given application.
Accordingly, the use of ultraviolet (UV) LEDs as the excitation source for fluorescent or phosphorescent lighting has become of greater interest. In theory, a single UV LED that produces an appropriate wavelength and frequency emission can produce a suitable white light emission from a complementary phosphor. Stated differently, by incorporating the phosphor, the single UV LED can produce the same white light that would otherwise require the use of separate red, green and blue LEDs. Present examples include potential back-lighting for liquid crystal display devices such as cell phone displays. Furthermore, the production of white light from single LEDs offers advantages in any number of applications including room and outdoor lighting. Therefore, producing and improving light emitting diodes that can emit in an efficient and satisfactory manner in the UV portion of the spectrum remains a desirable goal.